1. Field of the Invention
The present invention relates to optical pickup devices, and more particularly to read optical systems of optical pickup devices.
2. Background Art
The storage capacity of one layer of an optical disc is largely dependent on the wavelength of the semiconductor laser used and the numerical aperture (NA) of the objective lens. The shorter the wavelength of the semiconductor laser or the greater the NA, the greater the recording density can be made, and the more the capacity per layer can be increased. Optical disc drives currently on the market are mainly DVD (Digital Versatile Disc) drives that use red light of a wavelength in the vicinity of 650 nm and an objective lens with an NA of 0.6. However, as optical disc drives with recording densities surpassing those of DVDs, ones that have a blue-violet semiconductor laser of a wavelength in the vicinity of 405 nm as a light source and use an objective lens with an NA of 0.85 are also being shipped. As a method of further increasing currently achieved recording densities, one may contemplate shortening the wavelength used. However, difficulties are anticipated with respect to developing a semiconductor laser in the ultraviolet range, which is shorter than the aforementioned blue-violet, in addition to which degradation of polycarbonate disc substrates due to ultraviolet light is also anticipated. Further, with respect to increasing the NA of objective lenses, since the NA of objective lenses in air has a limit of 1, improving recording densities by way of the NA of objective lenses is also becoming difficult.
Under such circumstances, as a method of increasing the capacity of a single optical disc, the provision of dual layers is practiced. In Jpn. J. Appl. Phys., Vol. 42 (2003), pp. 956-960 (Non-Patent Document 1), a dual-layer phase change disc technology is presented. When a dual-layer optical disc is irradiated with laser light, the adjacent layer is simultaneously irradiated, and inter-layer crosstalk consequently becomes a problem. In order to mitigate this problem, inter-layer spacing is increased. Since laser light is focused, and layers other than the target layer (layer of interest) are offset from the focal plane of the laser light, crosstalk can be reduced.
On the other hand, when inter-layer spacing is widened, spherical aberration becomes a problem. The recording layer is embedded in polycarbonate whose refractive index is different from that of air, and its spherical aberration varies depending on the depth from the disc surface. The objective lens is designed in such a manner that its spherical aberration decreases for a given layer, and when the focal point of the laser light is moved to another layer, since the distance of the focal position from the surface is different, spherical aberration occurs. This aberration may be corrected by placing an expander lens optical system, which typically comprises two lenses, or a liquid crystal element before the objective lens. In other words, aberration may be corrected by varying the distance of the two lenses or the phase of the liquid crystal element. However, given that a certain compensatable range of the liquid crystal element or a movement mechanism for the lenses is to be realized within a small optical disc drive device, correcting a large spherical aberration proves difficult.
If multiple layers were to be provided in order to further increase capacity, the overall thickness of the multiple layers would be restricted due to the correction limit for spherical aberration. Thus, if there are many layers, inter-layer spacing becomes narrower. As a result, in actual multi-layer optical drive devices, inter-layer crosstalk would remain.
In order to reduce such crosstalk, ISOM/ODS '08, Technical Digest Post-deadline Papers, TD05-155 (2008) (Non-Patent Document 2) describes the use of the fact that when the reflection light from a multi-layer optical disc is focused with a lens, the respective focal positions of the reflection light from the target layer and the reflection light from the adjacent layer vary along the optical axis. Specifically, a grating is so disposed as to include the optical axis, and a reflecting mirror is disposed in the focal plane of the reflection light from the layer of interest. The reflection light from the adjacent layer is attenuated since it irradiates the grating. On the other hand, the reflection light from the layer of interest travels through the gap between the grating and the reflecting mirror. Consequently, it is able to return to a detection system without being attenuated. Thus, it becomes possible to reduce inter-layer crosstalk.
In addition, in Jpn. J. Appl. Phys., Vol. 45, No. 2B (2006), pp. 1174-1177 (Non-Patent Document 3), a tracking signal is obtained using one beam, and stray light from dual layers is prevented from affecting the tracking signal. By adopting a configuration in which the light at the center part of a grating disposed in the return path is detected at a place off the optical axis, stray light is prevented from being incident on a quadrant detector for detecting tracking signals disposed near the optical axis center.